The present invention is related generally to semiconductor fabrication and, more particularly, to a system and method for removal of photoresist following a contact etch, as part of integrated circuit manufacturing.
A schematic of a standard configuration of a photoresist (PR) stripping chamber and source is shown in FIG. 1. Gas coming from a set of flow controllers and valves 101, passes via tubing 102 to a plasma source 103. There, the gas becomes substantially dissociated (and weakly ionized) and then goes through a distribution/baffling system 104 into a wafer process enclosure 105. A pedestal 106 holds a wafer 107 which is to be stripped of PR and residues. On the wafer, radicals react with the PR and residues to form volatile or water-soluble reaction products that are then pumped out by ducts 108. This type of PR stripping chamber is widely used because it provides almost entirely neutral reactive species to strip the PR and does not subject the wafer to large amounts of charged particles that might damage the sensitive materials and layers used in making the integrated circuits. Such stripping systems are used for removing PR both during the early stages of IC fabrication, when the transistors are fabricated, as well as the later stages where the interconnecting metal lines are made to connect the transistors in a desired circuit pattern and to external circuits.
There are PR stripping applications done in the early stage of transistor fabrication on semiconductor wafers, including stripping after ion implantation and stripping after etching used to pattern layers or make openings. One of these stripping processes is done after etching through the first layer of insulation on the wafer to create openings to connect to the junctions and gate of the transistors. This latter etching process is called the “contact” etch. It is usually done in two steps, a dielectric etch and thin stop layer etch. PR removal processes for this application typically follow stop layer etch and may use a single step, but more commonly use two or more steps. It is submitted that current PR stripping processes for this application are likely to be inadequate to meet process requirements in the near future as the size of transistors continues to shrink, the thickness of critical layers on the wafer surface continues to decrease, and the materials used to make transistors is changed.
A standard contact etching process on wafers first opens holes through a silicon dioxide insulating layer covering the just-fabricated transistors. The etching process is stopped when it reaches a thin “stop” layer covering the silicide or metal materials in the junctions and gate. This stop layer is commonly formed of Silicon Nitride. but in future implementations, may be formed from other materials, and is used to protect a silicide junction underneath the insulating layer. Junction materials will be changing over the coming generations of semiconductor technology from cobalt silicide to nickel silicide for the 65 nm generation and possibly nickel-platinum silicide in the 32 nm generation of devices. At the same time, silicide thickness will be decreasing to 20 nm and then less. It is submitted that such thinner junctions using Nickel Silicide will suffer increased resistance with even a modest amount of chemical damage including oxidation of the silicide.
The silicon dioxide dielectric covering the stop layer must be etched to completion, though it has a different thickness above the gate than above source and drain. This etching process must be fast and aggressive to be cost-effective, so it uses more energetic ion bombardment to increase the rate and to get the desired vertical wall profile. Because of the damage this ion bombardment would cause to sensitive junctions and because of the varying thickness of the silicon dioxide layer, the etching process for silicon dioxide needs to be highly selective so that it does not penetrate the stop layer. Once the silicon dioxide is etched through, the wafers are put into a soft etching system that uses less energetic or no ion bombardment to gently etch the stop layer and uncover the silicide. At this point in the integration sequence, the un-etched, patterned PR layer still remains above the silicon dioxide, and polymer residues containing silicon are on the sidewalls both of PR and of the just-etched hole in the silicon dioxide. These need then to be removed without damaging the exposed silicide.
The conventional stripping and residue removal process following the contact and stop layer etching generally uses mostly oxygen gas fed to a plasma source, and may use wet chemicals or have a small addition of forming gas or fluorinated gas added in a second step to remove residues. However, most silicide materials used for junctions, including cobalt silicide and nickel silicide, are sensitive to oxygen and degraded in performance by it. Further, the fluorine in the residue removal step also attacks the silicide, causing degradation of ten or more Angstroms of material. In the past, including 130 nm IC technology, there has been sufficient thickness of silicide (or a protective sacrificial silicide used) that the material damaged by stripping and residue removal can afford to be lost without degrading circuit performance. Prior to deposition of an interconnect metal into the contact hole, the silicide surface is typically cleaned of damaged junction material by a sputter etching process.
Gas mixtures containing mainly oxygen have been the principal types of recipe used for all major stripping applications in transistor fabrication as part of IC manufacturing. Oxygen has been the gas of choice for more than 20 years because atomic oxygen reacts more strongly with organic polymers like PR than most other radicals so it gives the stripping process a very high rate that makes the process less expensive than when using other gases. Water vapor also produces high stripping rates in some types of systems but is more difficult to deliver in gaseous form at high flow rates as is oxygen. Higher reactivity of species makes stripping rates faster, and faster rates make stripping system productivity higher. Such high rates have been an economic necessity for competitive stripping for many years because photoresist thickness for older lithography technologies (preceding Deep Ultraviolet lithography at 248 nanometers) has been greater than a micron or more. Since there are typically twenty or more photoresist removal steps in the IC manufacturing process, high stripping rates, typically several microns per minute, are needed in stripping to keep IC costs low for mass-market products.
Gas mixtures having little or no oxygen or oxygen containing gas have been used with plasma-based systems since the early days of PR stripping where materials vulnerable to oxidation have been exposed on the wafer. One alternative to oxygen-based feed gases for stripping is hydrogen. In the early days of semiconductor IC fabs, hydrogen was employed as the main gas for stripping photoresist for some selected steps, during electrical interconnect formation, in the overall integrated circuit fabrication process to avoid oxidation of exposed interconnect metal on the wafers. Such an interconnect metal may include, for example, aluminum. This is currently the case for interconnect fabrication on integrated circuits where conducting wires on the wafer are made from copper. It is also true for other new materials such as low-k dielectrics. Consequently, processes employing high hydrogen concentration with no added oxygen are commonly used in the later stages of integrated circuit manufacture where copper and low-k dielectrics are exposed to the stripping reactive species (see for example, U.S. Pat. No. 6,630,406 issued to Walfried, et al.). In these processes, the hydrogen may also be used for reducing copper surfaces oxidized in previous steps.
Gas mixtures using hydrogen-containing gases with no oxygen have also been used for wafer surface treatments to avoid corrosion. In most cases, this was because metal surfaces or metal-containing residues left after stripping would form undesirable, typically oxide compounds on the surface of the wafer that would degrade the yield or performance of the IC. This and most other applications employing gas mixtures lacking oxygen have been steps in the fabrication of interconnects or wires between transistors.
Hydrogen has commonly been used as a minority additive in most of the PR stripping applications in the form of a dilute mixture in Nitrogen. This additive gas improved stripping rates slightly and greatly improved ability to wash residues away with a simple water rinse. It was also used in earlier generations of IC production by a few manufacturers to strip PR that had been exposed to ion implantation. The gas mixture having a small percentage (typically 3% to 4%) of hydrogen in nitrogen (called Forming Gas) has been a commonly used gas in semiconductor factories and safe for use in conventional PR stripping systems. Concerns such as electric charging, silicon damage or contamination, however, prevented the stripping processes using RIE with Forming Gas or Hydrogen from being commercially successful for PR Stripping and Residue removal. This was particularly the case for stripping as part of transistor fabrication, and was true even in the earlier generations of IC fabrication technology when devices were much less sensitive to such problems.
The present invention resolves the foregoing difficulties and concerns while providing still further advantages, as will be described.